This invention relates to curing of phenolic novolac resins at high temperatures. More particularly, this invention relates to the use of (lower) alkoxylated melamine-formaldehyde resins and (lower) alkoxylated benzoguanamine-formaldehyde resins as curing or hardening agents for novolacs at high temperature. The curing agents of this invention are also simply referred to herein as triazine hardeners.
In refractory applications, there is a need for a binder comprising a phenolic novolac and a curing agent wherein the binder remains uncured until high temperatures are attained. In the binders of this invention at least 50% of the cure and preferably at least 80% of the cure is accomplished above 392.degree. F. (200.degree. C.). This minimizes hardening and solidification of the binder in heated equipment and provides improved flow properties and prolonged workability for the binder.
There is also a need for such binder compositions which can be delivered to hot surfaces under pressure wherein the compositions further contain a particulate refractory and water wherein the hardener is not leached out from intimate contact with the other ingredients of the composition. This prevents or minimizes stratification on drying and curing of the composition.
Pitch as well as phenolic resins are used as binders for aggregates to prepare compositions for refractory tap hole application for blast furnaces. Tap hole mix users need a combination of early green strength and stability at 150.degree. F. (65.degree. C.) to 250.degree. F. (121.degree. C.) for up to several hours. Novolac with hexamethylenetetramine (hexa) has been used in such compositions. Also, hexa has been used as a hardener for novolac in the manufacture of electrodes. However, novolac/hexa binders lead to early cure whereas novolac without hexa or other hardener do not cure. Resoles are also unsatisfactory in such compositions because they cure at too low a temperature.
Melamine resins have also been mentioned as curing agents for novolacs or as thermosetting resins along with resoles, etc. in refractory compositions. However, the melamine resins used in this invention have advantageous properties in their use.
It is known that novolacs containing acids or thermally produced latent acids strongly catalyze the cure of melamine resins. However, the acid lowers the normal curing temperature of the novolac.
Apparatus and procedures exist for the robotic spraying of refractory material in a slurry of about 20% water for maintenance and repair of heated refractory surfaces. Illustrative of such a system is the TUNCAST Tundish Spray System of Foseco Inc.